1. Field of the Invention
The present invention relates to a method of manufacturing a shadow mask for a color picture tube and, more particularly, to a method of manufacturing a shadow mask for a color picture tube in which both surfaces of a thin metal plate are etched in separate steps to prepare to the shadow mask.
2. Description of the Prior Art
A shadow mask is mounted in a color picture tube to oppose a phosphor screen of three-color phosphors mounted on the inner surface of a panel. As shown in FIG. 1, this shadow mask comprises thin metal plate 1 having a large number of apertures 2 in a predetermined pattern so as to select electron beams incident on the phosphor screen. In order to reduce the electron beams impinging on the inner walls of apertures 2 and reflected toward the phosphor screen, an opening size of the apertures on the phosphor screen side is larger than that on the electron gun side.
A conventional shadow mask for a color picture tube used in a television set is manufactured by a method shown in FIGS. 2A to 2E.
As shown in FIG. 2A, a photoresist is applied to both cleaned surfaces of thin metal plate 1 and is dried to form resist films 4a and 4b. As shown in FIG. 2B, negative patterns 5a and 5b having aperture patterns of the shadow mask are brought into tight contact with resist films 4a and 4b, respectively, and exposure is performed. The exposed films are developed and the non-sensitized portions are removed, thereby forming resist patterns 4a and 4b shown in FIG. 2C. Reference numeral 6a denotes a large-sized exposed portion of thin metal plate 1; and 6b, a small-sized exposed portion of thin metal plate 1.
Both surfaces of thin metal plate 1 are simultaneously etched using resist films 4c and 4d as etching-resistive masks, thereby forming aperature 2 shown in FIG. 2D. Thereafter, resist patterns 4c and 4d are removed to obtain a shadow mask shown in FIG. 2D.
According to the above method, when the opening size d of aperture 2 on the electron gun side is decreased, its precision is degraded. It is difficult to obtain with excellent reproducibility a highly fine and precise shadow mask having smaller opening size d than the plate thickness for a display color picture tube requiring a fine and precise image.
A method of manufacturing such a highly fine and precise shadow mask, therefore, is exemplified by a two-step etching process shown in FIGS. 3A to 3F and disclosed in U.S. Pat. No. 4,689,114.
In the same manner as in steps shown in FIGS. 2A to 2C, resist films 4a and 4b are formed and negative patterns 5a and 5b are brought into tight contact therewith. Exposure and development are performed to from resist patterns 4c and 4d. As shown in FIG. 3A, protective film 8a is adhered to the surface of resist pattern 4c to protect etching the surface for the large opening size. In this state, resist pattern 4d for etching the surface for the small opening size is located on the lower side, and etching is performed to form small recess 9 (first etching). As shown in FIG. 3B, resist pattern 4d is removed. As shown in FIG. 3C, etching-resistive resin is coated on the thin metal plate surface with small recess 9 to form etching-resistive layer 10 so as to fill small recess 9. In addition, protective film 8b is adhered to etching-resistive layer 10.
In this state, resist pattern 4c is caused to face down and the thin metal plate surface with resist pattern 4c is etched to form large recess 11 which reaches the bottom of small recess 9, as shown in FIG. 3D (second etching). As shown in FIG. 3E, protective film 8b is removed, and resist pattern 4c and etching-resistive layer 10 are removed by an alkali solution, thereby obtaining a shadow mask shown in FIG. 5F.
According to the method shown in FIGS. 3A to 3F, etching-resistive layer 10 cannot often be removed completely due to its adhesion. In addition, a large amount of material of removed etching-resistive layer 10 is mixed in a resist film removing chamber or an alkali solution tank. This material is attached to aperture 2 to cause clogging. As a large amount of material of etching-resistive layer 10 is mixed in the alkali solution after the treatment, it is difficult to reproduce and reuse the alkali solution.